A backlight module includes a reflection sheet, a diffusion sheet, a prism sheet, a light guide plate and a light source, wherein the light guide plate is one of the most important elements of the backlight module. A light beam from the light source enters the light guide plate via a light entering surface of the light guide plate, and the light beam is totally reflected in the light guide plate and transmitted to an end of the light guide plate opposite to the light entering surface. A plurality of dot patterns are disposed on a bottom surface of the light guide plate, and the dot patterns are used to destroy the total reflection of the light beam, thereby guiding the light beam to a light emitting surface of the light guide plate.
China Patent No. 100507613C discloses a scattering sheet having a plurality of elliptic scattering patterns on a back surface of the scattering sheet. An incident light beam is scattered to transmit in at least two directions by the elliptic scattering patterns, thereby having different intensities. Taiwan Patent No. I356918B discloses an illuminating apparatus including a light source, a diffusion sheet, a brightness enhancing sheet, and an anisotropic diffusion sheet. China Patent No. 102162868B discloses an optical plate including a base and a plurality of optical patterns, wherein planar projections of the optical patterns are circle shapes or ellipse shapes. U.S. Pat. No. 7,628,526B2 discloses a light condensing sheet including a light condensing film and a plurality of elliptic light condensing dots formed on the light condensing film.
FIG. 1A is a cross-section schematic diagram of a conventional backlight module. Referring to FIG. 1A, the backlight module 1 includes a light source 11, an inverse prism sheet 12, a diffusion sheet 13, a light guide plate 14 and a reflection sheet 15. The light guide plate 14 has a lateral surface 141, a light emitting surface 142 and a bottom surface 143. The light source 11 is disposed beside the lateral surface 141 of the light guide plate 14. The inverse prism sheet 12 and the diffusion sheet 13 are both disposed above the light emitting surface 142 of the light guide plate 14. The reflection sheet 15 is disposed below the bottom surface 143 of the light guide plate 14.
A light beam from the light source 11 transmits in the light guide plate 14 and then emits from the light emitting surface 142. The reflection sheet 15 is used to reflect a portion of the light beam emitting from the bottom surface 143 back to the light guide plate 14 to increase light using efficiency. The light beam emitted from the light emitting surface 142 of the light guide plate 14 passes through the inverse prism sheet 12 and the diffusion sheet 13, and then emits toward a liquid crystal display (not shown).
After the light beam emits from the light guide plate 14 of the backlight module 1, a transmitting direction of the light beam is corrected by inverse prism structures 121 of the inverse prism sheet 12. Thus, a proportion of the light beam perpendicularly emitting from the light emitting surface 142 can be increased.
Referring to FIG. 1B, the backlight module 1 has a problem that a vertical viewing angle is too small. In detail, the vertical viewing angle of the backlight module 1 depends on the inverse prism sheet 12. The light beam provided by the backlight module 1 with the inverse prism sheet 12 has a problem that a horizontal viewing angle (direction X) and a vertical viewing angle (direction Y) are obviously different, and this may lead to visually discomfort when a user rotates a display apparatus having the backlight module 1 and watches the display apparatus from different directions. For this reason, how to solve the above-mentioned problems is one of the important issues for the person skilled in the art. If another inverse prism sheet is added and the inverse prism structures of the two inverse prism sheets are perpendicular with each other, the problem of the narrow viewing angle can't be solved, but the brightness may be decreased and interference stripes may be produced. If another diffusion sheet is added, the added diffusion sheet and the inverse prism sheet may result in interference stripes.